Circuit arrangement and thermionic valve for amplifying electrical oscillations



July 8, 194-1. BLACK ETAL' 2.248.804

CIRCUIT ARRANGEMENT AND THERMIONIC VALVE FOR,

AMPLIFYING ELECTRICAL OSCILLATIONS Filed. May 11, 1939 2 Sheets-Sheet 1//7 Mentors Attorney 1 July 8, 194-1.

H. BLACK ETAL 2248,804 CIRCUIT ARRANGEMENT AND THERMIONIG VALVE FORAMPLIFYING ELECTRICAL OSCILLA'I'IONS Filed May L1, 1939 2 Sheets-Sheet 21 7 20 l 5 7 v x 4 v 3 i6 K 2 18 v Fzg. 7.

at, 22 10% 7 l2 //7 manta/3 7 18 D. 819619 b A. Rae/1e Patente July 8,1941 S PATENT OFFICE CIRCUIT ARRANGEMENT AND THERMIONIC VALVE FORAMPLIFYING ELECTRICAL OSCILLATIONS Application May 11, 1939, Serial No.272,994 In Great Britain May'27, 1938 12 Claims.

This invention relates to circuit arrangements and thermionic valves foramplifying electrical oscillations wherein negative feedback is employedto reduce distortion in accordance with known underlying principles.

According to one feature of the invention a feedback voltage is derivedfrom one of the auxiliary electrodes in a thermionic discharge tubehaving an anode, a cathode and two or more auxiliary electrodes. Theimpedance connected between the feedback electrode and the cathode maybear the same relation to the impedance of the anode-cathode circuit asthe current to the anode does to the current to the feedback electrodefor the purpose hereinafter described. Alternatively the impedanceconnected between the feedback electrode and the cathode may beproportioned so that the harmonics developed therein are greater thanthose developed in the anode circuit. This impedance may also be madedependent on frequency, or may be proportioned to control the outputimpedance of the amplifier. There may be in addition, a feedback of avoltage proportional to the current in the anode circuit which may alsobe utilised to control the output impedance,

According to another feature a thermionic discharge tube, comprising ananode, a cathode, a control grid and at least one auxiliary electrode isso arranged that the ratio of the electron currents flowing to the anodeand to the said auxiliary electrode can be made substantially constantfor all working values of the control grid voltage.

Feedback according to the invention may be derived from an impedanceconnected to the said auxiliary electrode in such a thermionic dischargetube. Such auxiliary electrode may be an additional grid-like electrodeprovided next to the anode in a valve for example, of the pentode type.Feedback may alternatively or also be derived from the screen-gridcircuit of a valve having an anode, a cathode, a control electrode and ascreen-grid between the control grid and the anode.

Embodiments of the invention will now be described with reference to thedrawings. In these drawings:

Fig. 1 is a circuit diagram of an amplifier utilising the principles ofthe invention;

Figs. 2 to 4-. illustrate modifications in the output stage of theamplifier of Fig. 1;

Fig. 5 is a circuit diagram of an amplifier illustrating furthermodifications which may be made in the amplifier of Fig. 1; g I

Fig. 6 illustrates a thermionic valve constructed accordin to certainfeatures of the invention and a system of connections for utilising theValve as an amplifier in accordance with the invention; and

Fig. 7 illustrates certain modifications in the arrangement of Fi 6.

In the feedback amplifier arrangements illustrated in Figs. 1 to 5, thefeedback is taken from the screen grid circuit of the last tube and notfrom the amplifier output, as is usually done. The advantages of flatcharacteristic, gain stability and the facility o-f'impedance correctionare obtained with case.

Fig. 1 shows a three-stage amplifier embodying the principles describedin the present invention. Signals are led in from the transformer T1 andare applied to the grid of valve V1. This valve has a plate load R3 andthe screen is fed through the resistance B2, C1 being the usualdecoupling condenser. The bias is obtained from the resistance R4shunted by the condenser C2. From the plate of V1 the signals are ledthrough the coupling condenser C3 to the valve V2. This valve has aplate load Re, the screen being fed through R5 with the decouplingcondenser C4. The bias is obtained from the resistance R8 in the cathodelead, C5 being the usual decoupling condenser and R7 the leakresistance.From the plate of V signals are led through the coupling condenser C6 tothe grid of V3. As before, there is a load resistance R10, and thescreen is fed through a resistance R9. The bias is obtained from theresistance R12 and condenser C9, and is fed to the grid through aresistance R11. The output from the last tube is taken through thecondenser C3 to the output transformer T2 which steps down to therequired impedance for the line circuit. The screen grid circuit of thelast tube contains the resistance R13 in'series with the condenser C7.C7 has a negligible impedance at all relevant frequencies. The voltagegenerated across the resistance R13 is then fed back to the grid of thefirst tube by the connections as shown.

The valves used in the above amplifier may components being the sameexcept for the resistance R13 and R14 which will be referred to later.The method of obtaining feedback in this embodiment consists of thecombination of feedback from the screen circuit and from the platecircuit. Thus, considering the resistance R13, the alternating currentsin both screen-grid and plate circuits will flow through this resistanceand will hence be applied to the grid of the first tube. In additiverelation to this feedback voltage is the voltage generated across R14,which is in the screen-grid circuit as described with reference to Fig.1 above. With this arrangement gain stability and flatness ofcharacteristic etc., can be obtained to any desired degree providedstability conditions can be maintained. It also offers advantages in theway of adjustment of output impedance, as the current feedback due tothe resistance R13 will tend to raise the output im-' pedance lookingback into the amplifier, whereas the feedback from the screen circuitwill not so affect it, and it is therefore possible by a suitablecombination of the two types of feedback to obtain any required outputimpedance and amplitude. This variation in impedance may conveniently beobtained by tapping off a portion of R14 to obtain the required amountof feedback.

With regard to the harmonic reduction obtained with this type offeedback, it is essential for perfect results that the slope of thescreengrid characteristic shall be the same as that of the platecharacteristic. This may be achieved by suitable design of the valve, oralternatively a compromise may be made by inserting in the screen-gridcircuit a suitable load impedance to make the two characteristics moreidentical. This load may consist of a resistance a portion of which istapped off in order to provide the feedback voltage, or it may consistof one of the arrangements shown in Fig. 3 or 4.

With reference to Fig. 3 which again shows only the output stage of theamplifier of Fig. l, the transformer T2 is now replaced by athreewinding transformer, as shown in the figure. The feedback is thenobtained from the screengrid by means of a potentiometer Ru and R15which may also be used for impedance adjustment, and from the platecircuit by means of resistance R13 as previously described. The load forthe screen-grid circuit then consists of the third winding of thetransformer T2 and the impedance of this winding may be designed to havethe desired effect on the screen-grid characteristic of the tube. Withthis arrangement the power generated in the screen-grid circuit isconveyed through the line to the transformer T2.

With reference to Fig. 4, again the output stage only is shown, thedesignations being the same as before, but in this case the load in thescreengrid circuit consists of the transformer T3, which then provides asecond output from the amplifier and is useful in cases where monitoringetc., is desired. The feedback is obtained from the resistances R13, R14and R15 as before.

It will be noted that in a number of the circuits described aboveindividual feedback from the plateto the screen of the last tube is alsoobtained. Thus in Figs. 2, 3 and 4 the resistance R13 is common to bothplate and screen circuits and therefore provides a coupling for feedbackpurposes. There is no objection to this feedback from plate to screen asit has a further stabilising effect but in practice it will usually besmall compared with the overall feedback obtained in a three stageamplifier.

A further modification is shown in Fig. 5, in which it will be seen thatthe feedback voltage is applied to the screen-grid of the first tube.This arrangement has the advantage of giving somewhat improved phaseshift round the feedback loop. It will be obvious that by means of acombination of Figs. 1 and 5 it is possible to obtain feedback on bothcontrol grid and screengrid and this is often the most satisfactoryarrangement.

It is not necessary that tetrodes or pentodes be used in the stages ofthe amplifier other than that from which the feedback is obtained and inthe case of Fig. 5 other than that to which the feedback is applied. Theinvention may be used for amplifiers of any number of stages andfeedback may be used inside the amplifier in individual stages or roundthe complete circuit or any combination thereof without departing fromthe spirit of the invention.

The arrangements described with reference to Figs. 1 to 5 wherein thefeedback voltage is taken from the screen-grid circuit of a dischargetube and not from the output or anode circuit of the amplifier havecertain disadvantages as regards harmonic reduction and the efficientoperation of the discharge tube. Considering the question of harmonicreduction a disadvantage is that the harmonic reduction may not be equalto the gain reduction due to the fact that the screen-grid and anodecharacteristics are not usually similar unless special precautions aretaken. Also for efficient operation of the normal tetrode or pentocle itis usually desirable for the potential of the screen-grid to remainsubstantially constant.

In order to ensure that the feedback voltage is substantially a faithfulreplica of the output voltage, it is proposed according to anotherfeature of the invention to insert in the tube an additional electrodefor the purpose of providing the necessary feedback potential. Thiselectrode which may be of grid formation is preferably so designed andsituated in the discharge tube that the electron current flowing to itis a definite fraction of the electron current flowing to the anode. Inthe preferred arrangement the auxiliary feedback grid is situated nextto the anode and is so constructed that the area of the grid facing theelectron stream is a certain proportion of the area of the anode facingthe electron stream. This proportion may be of the order of 5% or less.

The operation of this part of the invention will now be described withreference to Fig. 6.

In this figure, I is a thermionic discharge tube consisting of cathode 2and anode I and four auxiliary electrodes 3, 4, 5 and 6. The anode isconnected to the high tension supply 22 via an impedance 9 which is highcompared with the load impedance l9 as seen through the transformer 20.The screen 4 is fed through the impedance 8 and is connected to thecathode through the condenser II which is of a magnitude sufficient tomaintain the said screengrid at a substantially constant potential. Theinput signals to the control grid 3 are applied through the transformer2| the necessary direct current bias being obtained by means of thecathode resistance 15 shunted by the condenser I4 which presents anegligible impedance at all relevant frequencies.

The feedback electrode 6 is connected to the high tension supply 22through the impedance 10 which is high compared with the impedances l6and I! in series. The feedback electrode 6 is also connected through theimpedances l2, l and I! to the earth point IS, the impedance ofcondenser [2 being negligible at all relevant frequencies. Theresistance I1 is included in the control grid circuit of the tube asshown in the figure.

In the simplest arrangement the sum of the impedances I6 and l! inseries bears the same relationship to the output load in the anodecircuit as the current to the anode I does to the current to thefeedback electrode 6. If the feedback grid 6 has been so constructedthat the current flowing to it is always a fixed proportion of thecurrent flowing to the anode 1 when they are maintained at the same D.C. potential, then the voltages developed across the anode load and theimpedances I6 and I! in series will be substantially equal. A portion ofthis voltage (determined by the relative values of i6 and I1) is fedback to the input circuit.

By a suitable Variation of the impedances Ii),

l6 and I! it is possible to increase the harmonic content of thefeedback grid-circuit as compared with the anode circuit, and by thismeans an increased harmonic reduction may be obtained for the same gainreduction.

Although the tube shown in Fig. 6 contains three grids apart from thefeedback electrode, it is not intended to restrict the invention to thistype of tube and it will be evident that tubes containing one, two,three or more grids apart from the feedback electrode may be employedwithout i,-

departing from the invention. It will also be obvious that the inventionmay be applied to amplifiers containing more than one tube and that thefeedback voltage may be applied to any stage thereof. It is also notintended to limit the invention to the particular method of applying thefeedback shown in Fig. 6, and it will be obvious to those skilled in theart that a number of modifications are possible without departing fromthe spirit of the invention.

A further advantage of the method of feedback described is that theapplication of such feedback will tend to reduce the output impedance ofthe amplifier and this is usually desirable with the types of valvesnormally used in current practice. Furthermore, by using a combinationof feedback from the special feedback grid and the anode circuit afurther adjustment of output impedance is possible. Such an arrangementis shown in Fig. 7 which is similar to Fig. 6 except for the resistance23 across which a feedback voltage dependent upon the anode current isdeveloped in the usual manner. This may be applied to the grid circuitin series with the feedback voltage obtained from the feedback grid. Thecurrent feedback from the anode circuit tends to raise the outputimpedance while that from the feedback electrode does not so increaseit, and it will therefore be seen that a very large amount of control ispossible.

The impedance connected to the auxiliary electrode may be such as tooffer a low impedance to one particular frequency but a high impedanceto multiples of this frequency. In this way harmonics generated in aradio frequency amplifier may be reduced without reducing the overallgain of the system to any marked extent.

One of the features of the method of feedback herein described is thatby suitable circuit arrangements one order of harmonics may be decreasedat the expense of the reduction in another order. In general, conditionsare so arranged that an optimum reduction in the totalharmjonic contentis obtained. It is well knownthat even order harmonics, and inparticular the second harmonic, may be reduced to a low value by usingvalves in the so-called push-pull arrangement. By using two valves ofthe type described above in a push-pull circuit any second orderharmonics are thereby reduced to a low level. By the adjustment of theauxiliary electrode circuit it is possible to obtain a marked reductionin the third harmonic and thus when using the push-pull arrangement verylow values of second and third harmonics are obtained.

What is claimed is:

1. Wave amplifier comprising an input circuit, an output circuit andelectron discharge apparatus coupled therebetween said apparatuscomprising a thermionic valve having a cathode, a control electrode and,beyond said control electrode from the cathode, an output electrode anda feedback electrode, separate load impedances, each effectivethroughout the operating frequency range of the amplifier, beingconnected respectively between said output electrode and said feedbackelectrode, and said cathode and means for feeding back to said inputcircuit a voltage derived from the load impedance connected to saidfeedback electrode to substantially decrease the gain throughout theoperating frequency range.

2. A feedback amplifier as claimed in claim 1, in which the impedanceconnected between the feedback electrode and the cathode bears the samerelation to the impedance connected between the anode and cathode as thecurrent to the anode does to the current to the feedback electrode.

3. A feedback amplifier as claimed in claim 1 in which the impedanceconnected between the feedback electrode and the cathode is soproportioned that the harmonics in the feedback electrode circuit aregreater than those developed in the anode circuit.

4. A feedback amplifier as claimed in claim 1 in which the impedanceconnected between the feedback electrode and the cathode is madefrequency dependent in order to provide a feedback voltage which isdependent upon the frequency of the input signals.

5. A feedback amplifier as claimed in claim I, further comprising meansfor feeding back an additional voltage proportional to the current inthe anode circuit.

6. A feedback amplifier as in claim 1, further comprising means forfeeding back an additional voltage proportioned to the current in theanode circuit, the output impedance of the amplifier being controlled bythe adjustment of the relative proportions of feedback obtained from theauxiliary feedback grid and the anode circuit.

7. A negative feedback amplifier consisting of at least one stage thelast stage of which comprises a valve having a screen-grid between thecontrol grid and anode, and means for obtaining the feedback voltagepartly from the screen grid circuit and partly from the anode circuit.

8. A negative feedback amplifier accorchng to claim 7 further comprisingmeans for adjusting the output impedance including means for correctlyproportioning the combined feedback obtained from both screen-grid andanode circuits.

9. A feedback amplifier according to claim 7 in which the impedancebetween the screen-grid and cathode is adjusted to make the distortionterms in the screen grid circuit approximately similar to thosecontained in the anode circuit.

10. A feedback amplifier according to claim 7, wherein the screen gridand anode circuits are connected through the windings of a three windingoutput transformer.

11. A feedback amplifier as in claim '7 further comprising means forobtaining a small auxiliary output for monitoring or other purposes fromthe circuit of the electrode from which the feedback voltage isobtained.

12. A feedback amplifier in accordance with claim 1 comprising apush-pull stage with feedback derived at least in part from auxiliaryelectrodes in the discharge tubes of said stage wherein the feedbackfrom each discharge tube is adjusted to give the optimum reduction inthe odd orders of harmonics.

DONALD HARRISON BLACK. ALLEMAN HOLLY ROCHE.

